When reading the levels in an oil depot, a technician noticed that the stock of foam concentrate had dropped by 29 m³. The technician decided to close the depot as a precaution whilst awaiting a resupply of foam concentrate, informed the inspection authorities for classified facilities, the Mayor and the Prefect, and had sampling and analyses carried out in the site’s network of 10 piezometers. The depot resumed its activity following delivery of the foam concentrate on 26/01.
The spilled product seeped into the ground then joined the fairly shallow water table (4 to 6 m) which flowed towards the Rhône river, located 500 m westward. This foam concentrate contained PFOS (perfluorooctane sulfonate), which is a toxic substance that is highly persistent and suspected as carcinogenic.
All local water users were identified, informed of the risk of contamination on 01/02 and their abstraction points were monitored on a monthly basis. The use of groundwater (human consumption, filling swimming pools, watering plants, new boreholes) was prohibited throughout the area affected. The operator financed the drinking water supply connection or supplied the residents who used the groundwater with bottles of water. Pollution was detected on the Rhône site but no residents who used the water for sanitary purposes were exposed to quantifiable concentrations.
A monitoring system with the installation of additional piezometers was installed: weekly monitoring of 23 piezometers over the operator’s 2 neighbouring sites (one on either side of the RN 7) and 5 others between the 2 sites and monthly monitoring of 11 specific piezometers. The source area was treated by “biostimulation”, i.e. aeration of the groundwater table through the injection of oxygenated enriched water to facilitate the degradation of biodegradable compounds (in particular glycol compounds). The water table was lowered by pumping at a flow rate limited to 95 m³/h to avoid damaging the banks of the RHONE river. The water collected was placed in tanks and initially destroyed externally, then subsequently treated on 6 filters (4 activated carbon and 2 sand filters) before being discharged. The PFOS content in the pumped raw water decreased steadily and the limit values were never reached. In April 2008, water table treatment was still underway and restrictions on the groundwater were maintained. The operator supplied the inspection authorities for classified facilities and health services with the water quality analysis results every month.
All of the fire system drain valves were opened to drain it prior to its frost protection. As the valve isolating the tank from the foam concentrate was faulty, the tank completely emptied and the product spread over the ground. The operator replaced all of the valves of this type and planned to progressively replace foam concentrates with PFOS on its sites.
